This application claims priority from an application entitled xe2x80x9cDevice and Method for Generating Quaternary Complex Quasi-Orthogonal Code and Performing Channel Spreading Using the Same in CDMA Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Jan. 11, Jan. 14 and Apr. 9, 1999 and respectively assigned Ser. Nos. 99-888, 99-1339 and 99-12563, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a spreading device and method for a mobile communication system, and in particular, to a method for generating complex quasi-orthogonal codes and an apparatus and method for spreading channel data using those generated complex quasi-orthogonal codes.
2. Description of the Related Art
In general, a CDMA (Code Division Multiple Access) mobile communication system performs channel separation using orthogonal codes in order to increase channel capacity. For example, a forward link specified by the IS-95/IS-95A standard separates the channels using the orthogonal codes. This channel separation method can also be applied to a reverse link through time alignment. In addition, a UMTS (Universal Mobile Terrestrial System) down link also spread the channels using the orthogonal codes.
FIG. 1 illustrates the IS-95/IS-95A forward link in which channels are separated by orthogonal codes. Referring to FIG. 1, channels are separated by associated orthogonal codes Wi (where i=0 to 63), respectively, which typically are Walsh codes. The IS-95/IS-95A forward link uses convolutional codes having a code rate R=1/2, employs BPSK (Binary Phase Shift Keying) modulation, and has a bandwidth of 1.2288 MHz. Accordingly, the number of available channels is 1.2288 MHz/(9.6 KHz*2)=64. That is, the IS-95/IS-95A forward link can separate channels using 64 Walsh codes.
As stated above, the number of available orthogonal codes depends on the employed modulation method and the minimum data rate. However, in future CDMA mobile communication systems, channels assigned to the users will increase in number in order to improve performance. To this end, future CDMA mobile communication systems will need to increase the channel capacity of traffic channels, pilot channels and control channels.
However, there are a limited number of available orthogonal codes the improved system can use. Therefore, any increase in channel capacity will be restricted due to the limitation on the number of available orthogonal codes. To solve this problem, it is desirable to generate quasi-orthogonal codes, which will have the least interference with the orthogonal codes and a variable data rate.
It is, therefore, an object of the present invention to provide a method for generating complex quasi-orthogonal codes having the least interference with orthogonal codes in a CDMA communication system using the orthogonal codes.
It is another object of the present invention to provide a method for generating complex quasi-orthogonal codes having a correlation of below: {square root over (L)} with orthogonal codes of length L by generating and applying complex quasi-orthogonal codes for QPSK (Phase Shift Keying) modulation.
It is still another object of the present invention to provide an apparatus and method for spreading channel data with the complex quasi-orthogonal code generated using a quasi-orthogonal code mask in a CDMA communication system.
It is still another object of the present invention to provide an apparatus and method for spreading channel data with complex quasi-orthogonal codes generated using a sign and a phase of quasi-orthogonal code in a CDMA communication system.
It is yet another object of the present invention to provide a method for generating quasi-orthogonal codes having the least interference with orthogonal codes thereby to increase a channel capacity in a CDMA communication system using the orthogonal codes.
It is yet another object of the present invention to provide a device and method for generating quasi-orthogonal sequences satisfying all the conditions of quasi-orthogonal codes in a CDMA communication system.
It is yet another object of the present invention to provide a column permutation method for generating quasi-orthogonal sequences satisfying all the conditions of quasi-orthogonal codes in a CDMA communication system.
It is yet another object of the present invention to provide quasi-orthogonal codes which can be expressed as a sign code and a phase code and satisfy conditions of the quasi orthogonal codes in a CDMA communication system.
It is yet another object of the present invention to provide a device and method for spreading and despreading a channel signal using the quasi-orthogonal code expressed as a sign code and a phase code in a CDMA communication system.
It is yet another object of the present invention to provide quasi-orthogonal codes which can be expressed as a specific walsh code used as a sign code and a phase code and satisfy conditions of the quasi-orthogonal codes in a CDMA communication system.
It is yet another object of the present invention to provide a device and method for spreading and despreading a channel signal using a quasi-orthogonal code expressed as a specific walsh code used as a sign code and a phase code in a CDMA communication system.
To achieve the above objects, a method for generating a complex quasi-orthogonal code for channel spreading in a CDMA communication system is provided. The method comprises generating an M-sequence having a length N and a specific sequence having a good full correlation property with the M-sequence; generating a predetermined number of other specific sequences by circularly shifting said specific sequence; generating predetermined number of other M-sequences by circularly shifting said M-sequence, and column permutating the circularly shifted specific sequences in a same method as a column permutation method for converting the generated M-sequences to Walsh orthogonal codes to generate candidate masks; generating quasi-orthogonal code representatives by operating the mask candidates and the Walsh orthogonal codes having the same length as the candidate masks; and selecting quasi-orthogonal code candidates satisfying a partial correlation between the Walsh orthogonal codes out of the generated quasi-orthogonal code representatives and a partial correlation between different quasi-orthogonal codes, and selecting masks pertinent to generating the selected quasi-orthogonal codes.